This invention relates to a voice demodulation system, and more particularly to an FM noise suppressor for obtaining an FM detection signal i.e. a composite stereo signal free from any jamming signal i.e. noise pulses.
A circuit for eliminating a jamming signal included in an FM detection signal is known in the art.
For example, in a circuit disclosed in British Pat. No. 1,279,756, there are used a jamming signal detector which detects a jamming signal in the FM detection signal, a gate circuit which is controlled by the detected signal from the detector and which receives the FM detection signal, a capacitor which receives an output from the gate circuit, and a parallel resonance circuit which resonates with a pilot signal of 19 KHz for FM stereo demodulation. In this circuit, during the period in which the gate circuit is disabled by the detected jamming signal, the capacitor holds a voice signal level immediately before the gate circuit is disabled. In addition, even when no pilot signal appears at the output of the gate circuit due to the disablement thereof, a signal of 19 KHz corresponding to the lost pilot signal is fed from the parallel resonance circuit. As a result, a signal less affected by the jamming signals is obtained from the FM detection signal including the jamming signals.
In the above construction for eliminating the jamming signals, however, the parallel resonance circuit must provide the 19 KHz signal in phase with the pilot signal being broadcast during the period in which the pilot signal is missing, and a high accuracy and stability are demanded of the resonance frequency. In order to make the resonance frequency exactly 19 KHz, an adjustment is necessary. Moreover, each time one jamming signal is detected, the gate circuit is disabled for a predetermined period of time irrespective of the duration of the jamming signal. Therefore, when the number of jamming signals detected per unit time is large, the period of time of the pilot signal for synchronization to be supplied to the parallel resonance circuit decreases drastically.